It is known that polyols such as polyesters and polyethers are basic intermediates in the preparation of polyurethanes. While polyformals have been known as polyurethane intermediates, they have not been widely employed in commercial applications. Polyformals are prepared by the reaction of aliphatic diols and formaldehyde under the influence of acid catalysts; thus, poly-1,6-hexaneformal is made by reaction of a slight excess of 1,6-hexanediol with formaldehyde in the presence of an acid catalyst such as p-toluene sulfonic acid, the reaction is carried out until substantially all the theoretical amount of water has been removed.
When this polyformal is used without further treatment to prepare a polyurethane prepolymer by reaction with an organic diisocyanate, the prepolymer becomes orange to red in color, which carries over to the subsequent reaction product. Moreover, on standing especially at temperatures above normal room temperatures, there is a distinct drop in the isocyanate content of the prepolymer and an increase in the viscosity oftentimes resulting in gelation. The reactions accompanying this instability appear to be complex and probably involve the formation of undesired by-products as a result of a secondary reaction. Thus, the prepolymers prepared from the polyformals are usually unstable.
The prepolymers are subsequently converted by chain-extension reaction to give elastomers, plastics and the like. Polyurethanes obtained by chain-extension with diols or triols have been found to be thermally unstable, losing much of their mechanical properties on exposure to elevated temperatures. Amine cured polymers, while ordinarily more thermally stable are badly colored. However, it is difficult to prepare a good polyurethane polymer by chain-extension of an unstable prepolymer with an aromatic diamine.
As hereinabove set forth, the polyformals are prepared by the condensation of an aliphatic diol with formaldehyde in the presence of an acid catalyst, with the elimination of water. In practice, the diol is used in slight excess over formaldehyde to insure hydroxyl group termination and to control the molecular weight. Although 1,6-hexanediol is of particular interest, other aliphatic diols can be used to make polyformals, such as those containing at least four atoms between the hydroxyl groups, preferably five atoms; these include, for example, 1,4-butanediol, 1,5-pentanediol, diethylene glycol, 1,7-heptanediol, and the like, as well as higher homologs. The formaldehyde is usually an aqueous solution of formaldehyde. Concentrated solutions of formaldehyde, that is, 37 percent or higher, tend to be unstable and polymerize. Thus, they usually contain a stabilizer or are kept hot to prevent polymerization. A concentrated solution of formaldehyde, 44 percent, can be made stable at room temperature by dissolving an equimolar amount of a diol into the solution. Effective use of such a stock solution can be advantageous in the preparation of polyformals since one need only add any excess amounts of diol and a catalyst to prepare a polyformal reaction mixture.
The polyformal condensation reaction is catalyzed by acids. The acids useful in preparing the polyformals of the present invention include Bronsted acids, that is species which can give up a proton to another species such as p-toluene sulfonic acid which is the preferred species of catalyst. Others include sulfuric acid, hydrochloric acid, phosphoric acid, methane sulfonic acid, napththalene sulfonic acid, and the like.
Ordinarily when the polyformal reaction is completed, the polymer is light yellow and has an apparent pH of about 3.4 to 3.6. When the polyformal is converted into a prepolymer by reaction with an organic diisocyanate, the color becomes orange to red, and on standing the isocyanate content decreases with an accompanying increase in the viscosity, frequently resulting in gelation.
The present invention represents the culmination of a long series of investigations, conducted largely by the inventors, directed to uncovering means for overcoming the inherent disadvantages of isocyanate terminated polyurethane prepolymers prepared from polyformals.
Accordingly, it is a primary object of the present invention to provide an improved process for polyformals which provide stable isocyanate terminated prepolymers and thermally stable polyurethane polymers.
It is another object of the invention to provide a process for removing the acid catalyst residues from polyformals.
It is yet another object of the invention to provide stable isocyanate terminated polyurethane prepolymers obtained from polyformals.
Generally then, it is an object of this invention to provide an improved process for preparing polyformals which possess none of the inherent disadvantages of the prior art polyformals.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.